Closes #43

https://bugs.musicpd.org/view.php?id=3876

The user unit omits the "ProtectKernelModules" setting which fails
with modular kernels:

 Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not permitted

It is unfortunate that systemd (version 232) is unable to reduce its
own capabilities, because this requires us to split system and user
units.

 https://bugs.musicpd.org/view.php?id=4608

https://bugs.musicpd.org/view.php?id=4560

https://bugs.musicpd.org/view.php?id=4525

A complete list which replaces the incomplete list in the mpd.conf
manpage.

MPD does not really take advantage of memory-mapped I/O by generating
data right into the ALSA buffer; using plain snd_pcm_mmap_writei() has
no advantage compared to snd_pcm_writei().  Let's kill this
non-feature.

These are other popular clients. In particular, VLC is available on
mobile devices.
Signed-off-by: Ben Boeckel <mathstuf@gmail.com>

Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729